1. Field of the Invention
Embodiments of the invention relate to assembly jigs and assembly methods for semiconductor devices, such as semiconductor modules.
2. Description of the Related Art
FIG. 12 is a schematic sectional view of a semiconductor module. This semiconductor module includes a packaging substrate 53 fixed on a cooling base 51 with solidified solder 52, semiconductor chips 55 fixed on the packaging substrate 53 with solidified solder 54, bonding wires 56 in connection with the semiconductor chips 55, a resin casing 58 having lead out terminals 57 attached thereon, and a filling material for example gel 59, filling inside the casing 58.
For packaging semiconductor modules, two methods are generally employed. In one of the two methods, semiconductor chips 55 and solder plates 54a before melting are positioned using an assembly jig 500, which can be a carbon jig, and soldered to a packaging substrate 53, as shown in FIGS. 16(a), 16(b) and 17(a), 17(b). In the other of the two methods, which is not depicted in the drawings, after printing solder paste on a packaging substrate 53 using a metal mask or the like, a semiconductor chip 55 is positioned and then soldered with the solder paste.
With the former method that uses an assembly jig 500, the assembly jig 500 must be designed in consideration of dimensions and scattering thereof of the semiconductor chip 55 and other parts, extent of distortion of the parts during the assembly process, and ease of assembly operation.
FIGS. 13(a), 13(b), 14(a), 14(b), 15(a) and 15(b) show construction of a conventional assembly jig for a semiconductor device, in which FIGS. 13(a) and 13(b) show a structure of an inner positioning piece, FIGS. 14(a) and 14(b) show a structure of an outer framework, and FIGS. 15(a) and 15(b) show construction of the whole assembly jig. Of these figures, FIGS. 13(a), 14(a), and 15(a) are plan views of essential parts, and FIGS. 13(b), 14(b), and 15(b) are sectional views taken along the lines X-X in FIGS. 13(a), 14(a), and 15(a), respectively.
Referring to FIGS. 13(a) and 13(b), an inner positioning piece 61 is composed of a flat plate 62 having openings 63. Semiconductor chips 55 and solder plates 54a, which are shown in FIG. 17(b), are inserted into the openings 63. A positioning piece here is a jig that is positioned by a frame at the outer periphery of the positioning piece and has an opening(s) (or a window(s)) inside the positioning piece for receiving and positioning a part(s), which may be a smaller positioning piece(s) or a semiconductor chip(s).
Referring to FIGS. 14(a) and 14(b), an outer framework 66 is composed of a frame 67 and metal pins 68. Disposed inside the inner surface side face 69 of the frame 67 are an inner positioning piece 61 and a packaging substrate 53 as shown in FIGS. 16(b) and 17(b). The metal pin 68 is inserted into a positioning hole 51a of a cooling base 51 to position the outer framework 66 on the cooling base 51 as shown in FIGS. 16(a), 16(b), 17(a), and 17(b).
Referring to FIGS. 15(a) and 15(b), the whole assembly jig 500 is composed of the inner positioning piece 61 and the outer framework 66.
FIGS. 16(a) and 16(b) show a construction with the assembly jig 500, the solder plate 52a, and the packaging substrate 53 positioned on the cooling base 51. The outer framework 66 is positioned and fixed on the cooling base 51 by the metal pins 68 inserted into the positioning holes 51a. The solder plate 52a and the packaging substrate 53 are put on the cooling base 51 and positioned by the metal pins 68, which are components of the outer framework 66. The inner positioning jig 61 is positioned by the inside wall surface 69 of the frame 67 composing the outer framework 66 and mounted on the packaging substrate 53.
FIGS. 17(a) and 17(b) show a construction with the packaging substrate 53, the assembly jig 500, and semiconductor chips 55 mounted on the cooling base 51, in which FIG. 17(a) is a plan view and FIG. 17(b) is a sectional view taken along the line X-X in FIG. 17(a). FIGS. 17(a) and 17(b) show the construction in which the solder plates 54a and the semiconductor chips 55 are inserted into the openings 63 of the inner positioning piece 61 shown in FIGS. 16(a) and 16(b) and positioned and mounted on the packaging substrate 53.
In the soldering step, the whole assembly shown in FIGS. 17(a) and 17(b) are put into a heating furnace (not shown in the drawings) and soldered in a reducing atmosphere at a high temperature without using flux, the whole assembly including the cooling base 51, the packaging substrate 53, the assembly jig 500, and the semiconductor chips 55. The packaging jig 500 is made from a material that does not contaminate the heating furnace, hardly deforms so as not to cause damages of cracks or the like in the mounted semiconductor chips and solder plates that become in contact with the packaging jig, and further is readily machined. Thus, carbon is generally used for the material of the assembly jig 500.
Japanese Unexamined Patent Application Publication No. 2007-194477 (also referred to herein as “Patent Document 1”) discloses a positioning jig comprising a first jig and a second jig, the latter being analogous to the inner positioning piece mentioned above. The first jig has a positioning hole (i.e., an opening) into which a solder sheet (i.e., a solder plate) and a semiconductor element (i.e., a semiconductor chip) are inserted. The positioning hole is disposed corresponding to a metal circuit (i.e., a conductive pattern) on a circuit board (i.e., a packaging substrate). The second jig is possible to be inserted into and removed from the positioning hole. The second jig has a pressing surface that opposes the metal circuit in the state of the second jig inserted into the positioning hole and presses the semiconductor element put on the solder sheet towards the circuit board. The second jig is positioned by the wall surface of the positioning hole so that the pressing surface opposes the metal circuit in the state of the second jig inserted into the positioning hole. The document asserts that such a construction well solders the semiconductor element to the joint place and forms a good solder fillet.
The purpose of the assembly jig in this document is to improve formation of a solder fillet. Fillet formation is assisted by making the jig directly in contact with the semiconductor chip and pressing the semiconductor chip. In order to obtain a good fillet configuration, the jig is made from stainless steel and heats the jig itself.
Japanese Unexamined Patent Application Publication No. 2008-270262 (also referred to herein as “Patent Document 2”) discloses a positioning jig in which a planar body, which is analogous to the inner positioning jig mentioned above, has protrusions protruding from the lower surface of the planar body toward an insulation substrate at places close to the outer periphery thereof by creating steps in the lower surface of the planar body within a range including a part of a positioning hole. Since the protrusions provided on the lower surface of the planar body touch or approach the upper surface of the insulation substrate close to the outer periphery thereof when the positioning jig is put on the insulation substrate convex upward, a solder foil and a heating element in the positioning hole are so disposed that movement in the positioning hole is restricted by the protrusions and thus, the solder foil and the heating element are mounted at a predetermined position. The document asserts that such a construction allows a semiconductor element to be mounted at a predetermined position on a curved insulation substrate without shifting the element.
In order to cope with the shift of the chip, the construction of this document comprises the parts analogous to the inner positioning piece and the outer framework, and in consideration of distortion of the packaging substrate protruding toward upper or front surface side of the packaging substrate due to heat in the soldering process, the inner positioning piece is cut in the central region thereof to absorb the protruding distortion of the packaging substrate.
Japanese Unexamined Patent Application Publication No. 2010-040881 (also referred to herein as “Patent Document 3”) discloses a positioning construction in which a carbon jig for positioning (i.e., an inner positioning piece) is disposed on the front surface side of an insulating circuit board and, a solder plate and a semiconductor chip are placed in the opening of the carbon jig. Then, the whole assembly is put into a heating furnace and heated up to a temperature higher than the melting point of the solder for example, about 300° C. to melt the solder. Thus, the semiconductor chip is mounted on the insulating circuit board. While the insulating circuit board is temporarily distorted in a configuration protruding toward rear surface side when heated, since the carbon jig has a stepped portion on the rear surface side so as to make the carbon jig thinner toward the direction separating from the opening, the degree of close attachment between the carbon jig and the insulating circuit board is enhanced, according to the assertion of the document.
The construction described in the document, in consideration of distortion of the packaging substrate, concaved in the front surface side thereof due to heat in the soldering process, has an inner positioning piece cut in the peripheral region thereof to absorb the concave distortion of the packaging substrate.
With the progress of performance improvement in semiconductor chips, reduction of size and thickness of a semiconductor chip has been recently in progress. The thickness of semiconductor chips has been reduced down to about 100 μm.
FIG. 18 shows a packaging substrate 53 curved at a temperature in the soldering process. The packaging substrate 53 curves due to thermal stress in the assembling process. This curved configuration creates a gap S between the packaging substrate surface 53a and the rear surface 61a (i.e., the rear surface of a flat plate 62) of the inner positioning piece 61 that is one of the components of the assembly jig. If the gap S becomes larger than a dimension T5 that is the sum of the thickness of the semiconductor chip 55 with an advanced thickness reduction and the thickness of the melted solder 54b, which is nearly equal to a thickness of the solder plate 54a, the semiconductor chip 55 should slip aside in this gap S, which causes difficulty in accurate positioning of the semiconductor chip.
The Patent Documents 1, 2, and 3 all do not disclose such a means for preventing the semiconductor chip from slipping aside in the gap that provides one chip-positioning piece for every one semiconductor chip and makes the gap between the assembly jig (i.e., the bottom surface of the chip positioning piece) and the thermally distorted packaging substrate surface be smaller than the sum of the thickness of the semiconductor chip and the thickness of the solder disposed under the semiconductor chip.
Further, in the construction disclosed in Patent Document 3, when the curvature of the concaved packaging substrate surface becomes large, the inner positioning piece in the document leaves the very large gap failing to absorb the large concaved curvature. Thus, the semiconductor chip slips aside in the gap. Accordingly, there are presently certain needs in the art.